Problem: The scale on a map is 1cm : 2km. If the distance between two cities is 20km, how far apart in cm are the two cities on the map?
Explanation: The scale means that every 1cm on the map corresponds to an actual distance of 2km. An actual distance of 20km is the same as 10 $\cdot$ 2km. The distance between the two cities on the map is 10 $\cdot$ 1cm, or 10cm.